The invention relates to a process and an apparatus of the type wherein a fluid such as a gas or vapor is used to treat a particulate type of contact material such as a catalyst, which is in a bed which is passing by gravity from one reaction or treating zone to another. One example of a process carried out in such an apparatus is catalyst regeneration. A specific example of a catalyst regeneration apparatus for use with a catalyst whose surface has been covered with coke during a catalytic reforming operation, and wherein the catalyst moves downward through a carbon burn-off section, through a halogenation section and into a drying section, is shown and described in Greenwood et al., U.S. Pat. No. 3,652,231, the subject matter of which is herein incorporated by reference. In the embodiment of FIG. 3 of the referenced patent, the catalyst moves downwardly as an annular bed in the annular space between a pair of spaced concentric perforated screens and is subjected initially to a combustion gas comprising a radially flowing recycle flue gas having a relatively low oxygen content and secondly, to a radially flowing second gas containing air, a halogen and steam. To achieve these flows, both the recycle flue gas and the second gas are circulated to the respective carbon burn-off and halogenation sections by separate fans or blowers. The catalyst then continues to move from the aforesaid annular space containing the halogenation section, as a descending bed, into a generally cylindrical drying section wherein it is contacted by a hot and dry air stream, or other suitable drying medium, before it exits the bottom of the apparatus. The drying medium is typically distributed by an arrangement of perforated distributor members including a central trunk with a plurality of laterally extending branches positioned generally uniformly in a horizontal plane near the bottom of the drying section. It is known that the mere presence of any pipes in the catalyst bed produces an obstruction in the downward flow path of the catalyst bed which can possibly accelerate attrition. However, I have found that any problem of possible attrition due to the presence of distributor pipes can be solved by uniformly feeding the gas to be distributed into the bottom of a cylindrical bed from the bottom of an annular plenum surrounding the bed. Such a construction is disclosed in my co-pending U.S. patent application, Ser. No. 780,788 filed Sept. 27, 1985, and assigned to a common assignee.
In regenerator units made in accordance with the teachings of the aforesaid U.S. Pat. No. 3,652,231, the quantity of air which can enter the drying section is controlled in response to the measured oxygen content of the combustion gas which is passed into the carbon burn-off section. As the air from the drying section passes up through the inside of the annular burn-off section, it becomes mixed with the flue gas which contains almost no oxygen after burning off the coke. The resultant gas mixture produces a recycle flue gas or combustion gas which desirably contains only about 0.8% oxygen. The oxygen content of this combustion gas must be very accurately controlled and thus, in such units, the amount of drying air must be accurately controlled. Too much oxygen in the combustion gas supplied to the coke covered catalyst would overheat and damage the catalyst while too little oxygen would allow too much coke to remain on the catalyst so that it would overheat when it was contacted by the air in the halogenation zone. Although it is essential to control the oxygen content of the combustion gas, it is also desirable that the amount of air entering the drying zone be controlled independently in order that the catalyst can be dried to the desired degree. It is also desirable that air from the drying zone be distributed to the halogenation zone since the use of such air can reduce the amount of additional heating required and can also utilize the pressure of the drying zone air which is supplied by the plant air supply. Since the use of a fan or blower to circulate air through the halogenation zone of the aforementioned prior art apparatus is expensive from both the standpoint of energy consumed as well as from the standpoint of capital equipment expense, it would be desirable to eliminate such fan.